Several kinds of structural configuration of photodetecting devices have been proposed. As disclosed for example in U.S. Pat. No. 6,380,568, a first known photodetecting device has a horizontal configuration, i.e., each photosensitive unit (or pixel) comprises on its surface, side by side, a photosensitive element and an electronic element, separated by a field isolation layer.
FIG. 1 a illustrates such a pixel 100, having a photodiode 10 (composed here, for example, of a p-type layer 10a and a n-type layer 10b), able to convert the received photons (hv) in electronic charges and, beside, a transistor 20 (surrounded by a field isolation layer 30) able to receive the electronic charges from the photodiode 10a. 
As shown on FIG. 1b, which represents a front view of the photoreceiver pixel surface 100, the latter comprises a photosensitive part 10 and a non-photosensitive part 25. Typically, the photosensitive surface represents 30% to 60% of the entire pixel surface. Consequently, the photosensing is not optimized, since a part of the photons arriving on the pixel are lost.
One solution to this would be to reduce the size of the electronic part 20. But such miniaturization has its own technical limits. Furthermore, even if the electronic part is drastically reduced in size, the surface of the pixel 100, in such a horizontal configuration, will not be dedicated to photosensing by 100%.
It was proposed to add a lens over each pixel for focusing the entire light, arriving on the pixel, to the photosensitive element; as disclosed in U.S. Pat. No. 6,040,591. But this solution is not desirable, due to its high cost and to its insufficient efficiency.
A second kind of known, photosensitive device is the so-called “back-illuminated” device. FIG. 2a illustrates this device which initially comprises a doped substrate (for instance a p+-type) covered by a top layer 1a having another type of doping (for instance a p−-type), and an electronic circuit layer 2 (including a plurality of transistors 20). The substrate is then selectively back-etched, such as to preserve the periphery 1b, and thus forming a central opening 15. The periphery 1a of the substrate then holds a thin structure comprising the top layer 1a and the circuit layer 2. This structure allows the thin top layer 1a to receive the photons coming from the “back” (i.e., passing through the opening 15) and delivering an electronic signal associated with this photon illumination to the transistors 20 included in the overlying circuit layer 2.
From FIGS. 2b and 2c, which represent respectively front views of the surface of a pixel 100 and of the opposite surface, it appears that this device does not have the drawbacks of the first photodetecting device, as the transistor 20 is not on the photoreceiving surface, the latter being then close to 100% dedicated to photosensing. However, the manufacturing of this second photodetecting device is high and implies the loss of an important part of the back substrate during the etch-back process.
A third known photodetecting device has a vertical configuration, i.e., the electronic part is buried under the photosensitive part, as disclosed for instance in European Patent Application 964,570 or in U.S. Pat. Nos. 6,831,264, 6,252,218, and 5,084,747. FIG. 3 illustrates such photodetecting device which is fabricated from a substrate 9 from which an electronic circuit layer 2 is formed (by crystal growth, doping, plating, deposition, . . . ). This circuit layer 2 usually comprises a circuit central part 22 including the transistors 20 of each pixel and a peripheral part 21 comprising electronic components dedicated to addressing (i.e., collecting the signals from the pixels and sending them to readout means). The circuit layer 2 is covered by an isolation layer 3, mainly of a dielectric material with a shielding 31 on the periphery for protecting the peripheral part 21 from illumination. The isolation layer 3 comprises electrical conductor channels (not shown), so called “via hole”, so as to electrically link the photosensitive part to the electronic part.
A doped photosensitive layer 1 (for instance a PiN structure composed of a p-type layer 1a, an intrinsic layer 1b and a n-type layer 1c) is formed on the isolation layer 3. Due to the non-crystallinity of the isolation layer 3, a crystal growth of the photosensitive layer 1 is not possible. So, the photosensitive layer 1 is formed by deposition, and has an amorphous structure. Now, an amorphous layer comprises many charges traps able to immobilize electrons and holes during a time, so called “time of relaxation”. These trapped charges slow down the reception and interfere with the next detections. The average number of electrons generated by incoming photons is then lower for non crystalline layers, from which it follows a lower efficiency for this type of device and leads to a less sensible detector.
Accordingly improvements in these type detectors are needed, and these are now provided by the present invention.